The aims of this project are: (1) to determine if interstitial levels of adenosine are increased by ischemic exercise in humans; (2) to test the hypothesis that endogenous adenosine contributes to the sensory stimulation and reflex sympathetic activation evoked by ischemic exercise in humans; (3) to test the hypothesis that endogenous adenosine acts as a local endogenous sympathetic modulator by blocking norepinephrine release in the forearm.